Underground mining of a coal body or seam is usually accomplished in two distinct phases. In the first phase, generally referred to as development mining or advance mining, the coal seam to be worked is divided and subdivided into several discrete areas by driving sets of entries into the coal seam according to a development pattern established for the proposed mine. Once the development phase is complete, the second phase, usually referred to as retreat mining, is initiated. Both mining phases, i.e., development and retreat, are usually carried out simultaneously in a given coal seam, with retreat mining occurring in previously developed areas and with development mining occurring in new areas of the seam.
The entry sets used to divide and subdivide the coal seam into discrete areas generally comprise a plurality of tunnels or entries that are oriented in generally parallel, spaced-apart relation. The entries in each entry set are connected together by a plurality of cross-cuts that are generally located transverse to each entry in the set. The arrangement is such that the entries and transverse cross-cuts together define a grid-like pattern of tunnels or entries that are separated by plurality of in-situ coal pillars. The pillars provide primary roof support in the mine.
Entry sets may be classified into different groups depending on their location and purpose within the mine. For example, the first entry set that is driven into the coal seam is usually referred to as the main entry set or simply "mains." The main entry set may comprise as many as eight or more individual entries in order to satisfy long-term mining requirements. Once the mains are established, a series of submain entries or cross entry sets may be driven into the coal seam at transverse angles with respect to the mains (usually 90.degree.) in order to further divide the seam into smaller and more workable sizes. Further subdivision of the coal seam may be effected by driving room entries at transverse angles (again usually 90.degree.) from the cross entries.
The number of entries comprising a given kind of entry set is usually determined by the requirements of ventilation, haulage, escapeways, and mine services such as power, water, and drainage. Other factors that can affect the size and number of entries comprising an entry set include the relative strength of the roof, the type and size of the mining and transportation equipment that is to be used and, of course, the nature and strength of the coal itself and the surrounding strata.
As was mentioned above, the entry sets that are driven into the coal seam during the development phase conform to the development pattern, which itself is based on the type of mining system to be used. For example, if the selected mining system is to be continuous or shortwall, entry development commonly follows a room and pillar plan. If the selected mining system is longwall, pairs of parallel entry sets, usually referred to as "gateroad entries" are driven into the coal seam from the mains or submains to define longwall panels. Each longwall panel is then removed by a suitable longwall shearing machine.
Regardless of the type of mining system that is to be used, e.g., continuous, shortwall or longwall, the entry sets that are driven into the coal seam during the development phase nearly always rely on in-situ coal pillars for primary roof support. As was briefly mentioned above, in-situ coal pillars are large blocks of coal that are defined by the various entries and cross-cuts that comprise the entry set. While the size of the pillars depends on the particular mine in which they are used, they tend to be 40 or so feet wide and may have lengths of 100 feet or more.
Since the in-situ pillars provide the primary support for the entry roof, it is imperative for mine safety that the pillars be of a size sufficient to adequately support the roof all times, both during the development phase and the retreat phase. A highly stressed pillar can be unstable and, if the compressive stress exceeds the yield strength of the pillar, the pillar can fail, with catastrophic consequences. Consequently, early mining operations tended to follow development plans that called for oversized pillars to provide wide safety margins. Unfortunately, such oversized pillars also represented a substantial amount of coal which, under most circumstances, could never be recovered.
Over the years various methods and devices have been developed in attempts to allow mine operators to better determine the compressive stress on pillars, so that they can be made as small as possible while still providing adequate safety margins. One such method has been to install strain gauge transducers deep within the pillars to allow the compressive stress to be monitored directly. While such strain gauge transducers do allow the compressive stress to be more accurately determined, they are not without their disadvantages. For example, since the strain gauge transducers must be placed deep within the pillar, it is necessary to first drill a hole into the pillar before the transducer can be installed. Also, once installed, it is not practical to remove the strain gauge transducer. As a result, the transducers are eventually lost to the gob.
Consequently, a need exists for a method and apparatus for determining the compressive stress in pillars that does not rely on the use of strain gauge transducers, with all their associated disadvantages. Ideally, such a method and apparatus would allow the compressive stress to be accurately determined for any pillar and during any phase of the mining operation. Additional advantages could be realized if the associated hardware could be re-used and not lost to the gob.